Hook up listers implemented with the new pattern to the CLI

We stop depending on the ListerBase implementation. The main hoop we're jumping
through is the config override mechanism in swh.lister.get_lister, as it's
really specifc to the ListerBase `override_config` argument, which is dropped in
pattern.Lister (in favor of explicit arguments at lister instantiation).

We implement a small shim in swh.lister.pattern.Lister to give
backwards-compatibility for the new pattern to get_lister.

This generic configuration override mechanism will probably be completely
removed when the configuration mechanism is reworked. We'll see.

swh/lister/pattern.py

@@ -6,7 +6,7 @@
 from dataclasses import dataclass
 from typing import Any, Dict, Generic, Iterable, Iterator, List, Optional, TypeVar
 
-from swh.scheduler import model
+from swh.scheduler import get_scheduler, model
 from swh.scheduler.interface import SchedulerInterface
 
 
@@ -222,3 +222,27 @@ class Lister(Generic[StateType, PageType]):
         """
         ret = self.scheduler.record_listed_origins(origins)
         return len(ret)
+
+    @classmethod
+    def from_config(cls, scheduler: Dict[str, Any], **config: Any):
+        """Instantiate a lister from a configuration dict.
+
+        This is basically a backwards-compatibility shim for the CLI.
+
+        Args:
+          scheduler: instantiation config for the scheduler
+          config: the configuration dict for the lister, with the following keys:
+            - credentials (optional): credentials list for the scheduler
+            - any other kwargs passed to the lister.
+
+        Returns:
+          the instantiated lister
+        """
+        # Drop the legacy config keys which aren't used for this generation of listers.
+        for legacy_key in ("storage", "lister", "celery"):
+            config.pop(legacy_key, None)
+
+        # Instantiate the scheduler
+        scheduler_instance = get_scheduler(**scheduler)
+
+        return cls(scheduler=scheduler_instance, **config)